1. Field of the Invention
The invention relates to a combination reforming-isomerization process for upgrading naphtha feedstocks.
2. Description of the Prior Art
Catalytic reforming of naphtha converts low octane components of the naphtha into higher octane compounds by a combination of dehydrogenation, dehydrocyclization, and isomerization. Such conversion to increase octane rating is commonly called "upgrading". One of the undesirable side reactions of catalytic reforming is cracking of high boiling components of the naphtha into low octane C.sub.5 and C.sub.6 hydrocarbons. The presence of such low octane hydrocarbons lowers the overall octane rating of the reformate. This situation has been remedied by separating the C.sub.5 and C.sub.6 component of the reformate and subjecting it to a separate isomerization. Processes that involve separate reforming and isomerization are called "combination" processes.
The patent literature describes several combination reforming-isomerization processes. U.S. Pat. No. 2,263,026 describes a combination process in which a heavy straight run naphtha is vaporized and fed to a dehydrogenation reactor at 750.degree. F. to 900.degree. F. where the saturated paraffins in the naphtha are converted to olefins and aromatics. The dehydrogenated naphtha is then fed to an isomerization reactor at 600.degree. F. to 750.degree. F. where the olefins are converted into branched or cyclic isomers.
U.S. Pat. No. 2,946,736 teaches a process for upgrading a full boiling naphtha in which the naphtha is first fractionated into a 180.degree. F.- fraction and a 180.degree. F.+ fraction. The 180.degree. F.+ fraction is catalytically reformed and the components of the reformate boiling below 180.degree. F. are removed from the reformate. The two 180.degree. F.- fractions--the one from the naphtha feed and the one from the reformate--are combined and the n-C.sub.5 and n-C.sub.6 are removed from the combined 180.degree. F.- fractions. The n-C.sub.5 and n-C.sub.6 are then upgraded by isomerization.
U.S. Pat. No. 3,287,253 describes a three-stage naphtha reforming process in which each stage involves a different catalyst that selectively promotes one or more reforming reactions. The first stage promotes dehydrogenation of naphthenes. The second promotes dehydrogenation of naphthenes and isomerization of straight chain and cyclic paraffins. The third stage promotes dehydrocyclization of paraffins.
U.S. Pat. No. 3,502,570 teaches a process in which naphtha is first subjected to a sulfur-modified reforming process. The reformate is then fractionated into four fractions: C.sub.1 -C.sub.4 ; C.sub.5 -C.sub.6 ; C.sub.7 ; and C.sub.8 +. The C.sub.5 -C.sub.6 fraction is isomerized and the isomerizate is blended with the C.sub.8 + fraction.